Invoke Recovery mode while the Mac is starting up by pressing together the Command + R keys.

Open your OS X Utilities. From your OS X Utilities menu, click Restore from Time Machine Backup. We are assuming that you had made a complete backup before installing the macOS beta on your machine.

Select your Time Machine backup disk

Select the backup you want to restore from

Click Continue and follow the instructions to reinstall OS X and your backed up files.

Check Your System

At this point, you have erased macOS Sierra and re-installed El Capitan. Restart your Mac and you should be all set to go.

In the case, you run into issues in starting your Mac or Macbook after the El Capitan re-install, you may have to check some of your kext settings if you hadn’t already taken care of it prior to the beta install.